The invention relates to a method for compressing and decompressing video data.
Videos generate large datasets. To effectively transfer and store these datasets, it is practical to compress them.
According to today's state of the art, video signals are recorded and reproduced in a rapid sequence of individual images. In television (PAL standard) there are 25 images per second, or 50 half-images. With digital recordings there are approximately 30 images per second. Each image is broken down into lines, and transferred first sequentially.
Previous compression methods are based essentially on the reduction of the resolution, of the color depth and the lowering of the number of images per second. With digital compression, e.g. the MPEG method, instead of complete images essentially the differential images, i.e. the differences of the individual image points (pixels) compared to the previous image, are transferred in place of the complete images. The latest standard for video coding is MPEG-4. MPEG is the abbreviation for “Motion Pictures Expert Group”. File formats from this group and methods for space-saving compression and storage of video or multimedia data (video, image and sound data) are defined in high quality. The MPEG standard meanwhile is subdivided into MPEG-1, MPEG-2, MPEG-3 and MPEG-4, whereby the MPEG-3 standard has been integrated into MPEG-2.
To be able to process and transport the huge amounts of data from films with “normal” computers, only the changes from the previous image are stored. The MPEG format stores so-called intra-frames at regular intervals of typically twelve images. Intra-frames are JPEG-compressed single images. The images between these I-frames are, if possible, not completely stored. Instead, MPEG stores images in a manner in which one can regain them by shifting parts from preceding or succeeding images. For this purpose “predicted frames” and b-frames (bi-directional frames) are used. However, since this never works perfectly, the remaining deviation per image is additionally stored JPEG-coded. With this method it is possible to reduce the data expenditure for a video film by about 99%. The potential compression goes all the way to 200:1.
MPEG-1 was designed for fluid video playbacks. The MPEG-1 compression or decompression was originally a hardware-dependent method. However, in the meantime, thanks to speedy processors, software decompression is also possible.
The essential difference between MPEG-1 and MPEG-2 consists in the fact that MPEG-2 can work better with interlaced scanning, the method used with television. The secret of MPEG-2 lies in the compression to the highest level of quality, so that film material can be processed and edited almost 1 to 1 in studio quality. Consequently, MPEG-2 established itself as a standard. With a pure 1 frame coding MPEG-2 can even be used in splicing. The part of the MPEG-3 standard that was provided for high definition TV quality (HDTV) had meanwhile been implemented in the MPEG-2 standard.
MPEG-4 is a further development of the MPEG-2 format and has been in development since 1996. Although MPEG-4 was originally intended as a coding standard for audiovisual data with very low bit rates, the development served far more purposes than merely streaming of linear media data with Internet and wireless applications. MPEG-4 for example provides efficient mechanisms for compression and distribution of interactive media contents. Moreover, MPEG-4 has 3-D potentials in order to visualize artificial intelligence or present avatars, e.g. in the course of video conferences. The compression rate with MPEG-4 is higher than with MPEG-2, whereby “sprites” can be compressed better, because the coding mechanism has more time at its disposal for this purpose. In the process it is even possible to switch to wavelets. The scripting language makes it possible in a few bytes to perform operations such as “translation” significantly faster than the digitized compressed form of the same operation would make possible. With the help of these “sprites” it is possible to move any number of contoured still images over moving pictures.
The object of the invention lies in the creation of a method for compressing video data which allows a simple and flexible adaptation to different transfer rates or transmission bandwidth, image resolutions and display sizes.
Advantageous designs and enhancements of the invention are specified in the dependent claims.